Gas turbines are well-known in the art. It is an ongoing quest within the gas turbine field to increase the thermal efficiency of the gas turbine cycle. One way this has been accomplished is via the development of increasingly temperature-resistant materials, or materials that are able to maintain their structural integrity over time at high temperatures. For this reason, the hot gas path components of gas turbine engines are often formed from superalloy materials. The term “superalloy” is used herein as it is commonly used in the art to refer to a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, e.g., >1000° C.
Despite the improvement in materials, the push to drive gas turbine operating temperatures even higher to increase engine efficiency has led to the application of a protective coating to the component surface. In some instances, this protective coating comprises both an oxidation resistant metallic bond coat (e.g., an MCrAlY alloy as is known in the art) and a thermally insulating thermal barrier coating (TBC). In such case, the bond coat further improves adherence of the TBC to the component surface. In other instances, the protective coating merely includes the bond coat, which may be applied to provide an oxidation resistant coating to the component with a degree of thermal protection. In either case, current bond coat application techniques are characterized by the ongoing loss of the bond coat over the service life of the component and/or by limited thicknesses. For example, it has been found that thermally sprayed bond coats can only provide coatings of limited thickness. As the coating thickness increases upon deposition of the bond coat material, compressive forces increase which leads to breaking away or depletion of the bond coat material.
In addition, in the repair of service run components having a bond coat with damage to the underlying substrate, the protective coating is typically chemically stripped from the coating. Thereafter, the underlying substrate is repaired utilizing a brazing or welding technique as known in the art. Next, the protective coating (bond coat or bond coat and TBC) is applied to the component. The sum of all these steps results in significant cost and time, which often leads to disposal of the part rather than bear the expense of repair. Accordingly, improved bond coat application techniques are needed for the repair of service run components which reduce cost and time and result in improved oxidation resistance and reduced material depletion.